In 1990, the U.S. government issued the Clean Air Act Amendment (CAAA) listing 189 hazardous air pollutants (HAPS). Hydrocarbons are among the contaminants listed, most notably the aromatics (benzene, toluene, ethyl benzene, and xylene) a.k.a. BTEXs causing a tremendous impact to the triethylene glycol (a.k.a. “TEG”) dehydration industry. All other light hydrocarbons are considered contaminants and combined with BTEX's, are classified as volatile organic compounds (VOC's) or total hydrocarbons (THC's).
Contaminants:
Glycol, as a hydrocarbon derivative (related alcohol compound), has an affinity for absorbing hydrocarbons. Present art glycol dehydration systems utilize a contactor tower to exchange natural gas and glycol under pressure. When natural gas is being dehydrated in a contactor, a certain percentage of it is absorbed into the glycol. This includes BTEX's and other hydrocarbons (e.g. paraffins, olefins, etc.). BTEX's are a primary concern because they have been classified as a carcinogen. Typical BTEX concentration in natural gas streams have been reported to range from <100 to 1000 ppmv.
Contaminant Sources:
In a typical present art glycol dehydration system there are three main sources for atmospheric contaminants:
Reboiler Exhaust: Reboiler exhaust of present art systems (stack) releases combustion byproducts to the atmosphere. The potential CAAA listed contaminants from these byproducts are NOx, CO, and VOC's.
Flash Tank Vent Gas: The glycol regeneration unit flash tank of present systems is designed to separate natural gas from the rich glycol stream. This occurs when a rich glycol stream pressure is decreased from a contactor operating pressure to a flash tank pressure (typically 50–75 psig). The potential CAAA contaminants are VOC's. The vent gas from the flash tank can be transported to a reboiler still column for eventual oxidation.
Reboiler Still Effluent:
In present art systems, a glycol regeneration reboiler's primary purpose is to vaporize absorbed water to regenerate glycol. There is, however, still some of the hydrocarbons present in the glycol solution after leaving the flash tank which will vaporize in the reboiler. All of the vaporized products exit the glycol still column and are typically released to the atmosphere. If stripping gas is required, it also exits the still column with vaporized water and hydrocarbons.
Attempts to reduce or eliminate the emission of undesirable compounds associated with glycol dehydration units and processes are known and represented in the art. For example:
U.S. Pat. No. 5,234,552 issued on Aug. 10, 1993 to Robert McGrew and John P. Broussard discloses a vapor condenser connected to a glycol reboiler to prevent emissions of aromatic compounds from glycol dehydration from escaping into the atmosphere. Steam and vaporized hydrocarbons are directed into the vapor condenser where they are sprayed with 80° F. or below water. The water spray cools the steam and vaporized hydrocarbons so that a substantial portion of the vapors are changed to a liquid phase which is collected in an accumulator located below the vapor condenser. A water jacket surrounding a central cylinder of the vapor condenser tends to keep the condensed vapors in the liquid phase. The remaining vaporized hydrocarbons which are not condensed are drawn out of the accumulator and burned in a burner connected to the glycol reboiler. The steam and vaporized hydrocarbons in the glycol reboiler have an initial temperature in the range of 350° to 400° F.
U.S. Pat. No. 5,520,723 issued on May 28, 1996 to Robert Jones Jr. discloses a method and system for reducing emissions from glycol dehydrators which employ a vapor-liquid contactor operated under ambient conditions to treat the organic vapors and liquids exiting from a condenser attached to the vapor vent of the glycol reboiler. A stream of the organic liquids is passed to the top of the contactor and allowed to descend in counter-current relation to the upward moving gas introduced at the bottom portion of the contactor. Liquids containing a relatively high content of hydrocarbons may be recovered from the bottom of the contactor, and vapors having a reduced content of organic emissions can be emitted directly to the atmosphere from the top of the contactor.
U.S. Pat. No. 5,824,836 issued on Oct. 20, 1998 to James Becquet discloses a system for lowering the ambient temperature of a vapor being produced from a glycol dehydrator unit, as well as a system for reducing the emissions of BTEX. Generally, the system comprises a condenser for condensing the vapor into a fluid phase and a gas phase, and a storage tank, fluidly connected to the outlet of the condenser. The system will also contain a pump member, operatively associated with the storage tank, adapted for pumping the fluid phase from the storage tank; and, an activating member adapted for activating the pump means after the fluid phase reaches a predetermined level within the storage tank. The system will include as part of the condenser member a turbine associated with a roof, with the roof being generally positioned over the condenser coils so as to shade the condenser coils. A method of recovering hydrocarbons from a vapor phase as well as lowering the ambient temperature is also disclosed.
U.S. Pat. No. 5,766,313 issued on Jun. 16, 1998 to Rodney Heath discloses an apparatus for treating emissions from a reboiler used to remove glycol from water laden glycol wherein the emissions are condensed; pressurized and separated so that hydrocarbon vapors may be directed to a burner used to supply heat to a reboiler. Control apparatus is provided to combine fuel gas and the pressurized hydrocarbon vapors as needed to supply all of the fuel required by the burner. Also, separate apparatus is provided to control the movement of dry glycol in the apparatus and to pressurize the condensed emissions.
U.S. Pat. No. 5,352,115 issued on Oct. 4, 1994 to Joseph M. Klobucar, subsequently assigned to Durr Industries, Inc. discloses a regenerative thermal oxidizer having regenerative heat exchangers including a heat exchange column formed of a body which defines at least one entire flow passage through the heat exchanger. The heat exchanger column assists in purging residual gas to be cleaned from the heat exchanger prior to that regenerative heat exchanger moving into a mode where it receives the cleaned gas. This reduces inadvertent emissions of gas to be cleaned to the environment. In one embodiment, a monolithic body includes all of the flow passages. In a second embodiment, a plurality of blocks are utilized to form the heat exchanger column. In a third embodiment, a number of cylindrical tubes are utilized. The heat exchanger columns preferably have 70–80% of their surface area used as the flow passages.
U.S. Pat. No. 5,221,523 issued on Jun. 22, 1993 to Bert M. Miles and Gary W. Sams, subsequently assigned to National Tank Company, discloses a system for controlling organic contaminants released from a regenerator during the process of liquid dehydration of natural gas where contaminants are vaporized mixture from the regenerator to a heater, heating the vaporized mixture in the heater, conducting the vaporized mixture from the heater to a liquid collection chamber where suspended liquid particles are separated out from the vaporized mixture, drawing the vaporized mixture from the liquid chamber using fuel gas as an aspirator, mixing atmospheric air with the vaporized mixture, and combusting the vaporized mixture in order to incinerate the vaporized mixture.
Given the limitations of the present art, what is needed is an improved method and article of manufacture to facilitate a one-step process which completely, or near completely, oxidizes effluent vapors emanated from a glycol based regeneration boiler. The present invention improves upon the present art by so doing and further eliminates the need to dispose of contaminant residuals. The invention additionally improves upon the art by eliminating the need to contend with the inefficiencies and potential failure of intermediate separators and pumps employed with prior art systems. The present art purportedly condenses a majority of still effluents. However, such condensed liquid is itself a contaminant which must be disposed of in a controlled manner. By contrast, the controlled oxidizing chamber of the present invention insures oxidation of undesirable effluents is complete and allows the unit to function in a superior manner to flare systems of the present art which present design structures of considerably less combustion efficiency.
There also remains a need to utilize energy generated in the oxidation stage as heat energy for the regeneration reboiler.